Marine seismic surveys are usually conducted using a seismic vessel towing seismic sources and a number of parallel streamer cables with hydrophones. The seismic streamer cables may be of several thousand meters length. The parallel streamer cables are usually towed along straight lines over the prospective geological formation. The streamer cables are spanned out by starboard side (SS) and port side (PS) deflectors which in turn are towed by the ship. In order to achieve coverage of the desired geographical area the parallel seismic cables should be towed several times along parallel, oppositely directed course lines. It is rational to turn at the end of each course line and conduct the next course line on an opposite course. An uncontrolled turn at the end of a course line with an unadjusted deflector will, for the outer deflector in the U-turn process, incur an increased load on the towing line, the bridle block, the bridle lines and the deflector's connection points, and on the deflector itself. The inner deflector in the U-turn process should have a certain speed in order to pull to a desired degree, and a deflector with a fixed position may exert an insufficient pulling force.
A commonly used procedure is to pre-set the deflectors' angle of attack manually by manually adjusting the bridle line lengths or to adjust their connectors positions on the bridle block inwardly or outwardly, before the launching of the deflectors. However, such a manually adapted bridle line length does not allow controlling the deflector's heading before, during and after the U-turn: the deflector is fixedly adjusted.